This invention relates to the production of isocyanates having at least two --N.dbd.C.dbd.O groups. In one aspect, this invention relates to the production of diisocyanates. This invention also relates to the production of carbamate esters having at least two ##STR1## groups. In another aspect, this invention relates to the production of bis(carbamate esters). This inventions further relates to polyamides prepared from such isocyanates.
Commercially, the phosgenation of primary amines is by far the most widely used method for producing isocyanates. The use of phosgene, however, has several disadvantages. The phosgenation route is long, energy intensive and requires handling highly corrosive materials, e.g., hydrogen chloride, chlorine, sulfuric acid and nitric acid, and highly toxic reagents and intermediates, e.g., phosgene and chlorine. Furthermore, the phosgenation route requires use of process equipment which can withstand high temperatures and highly corrosive conditions resulting in increased capital costs. For example, p-phenylenediisocyanate is prepared via the phosgenation of p-phenylenediamine. This route produces 4 moles of hydrogen chloride per mole of diisocyanate and requires the handling of both gaseous phosgene and gaseous hydrogen chloride. In addition, the product is frequently contaminated with the intermediate carbamyl chloride or symmetric urea as a result of incomplete conversion of the diamine.
Carbamate esters are typically prepared by reaction of an isocyanate, prepared as above, with an alcohol. This process has the same disadvantages as discussed above for the production of isocyanates. Carbamate esters can also be prepared via reductive carbonylation of an amine utilizing high pressures of toxic carbon monoxide and high temperatures in the presence of an alcohol and a catalyst.
Substituted aromatic amines having at least two --NH.sub.2 groups, e.g., p-phenylenediamine, are typically prepared via a halide route which is disadvantageous in that halide appears in the waste stream and must be disposed of at considerable expense. p-Phenylenediamine is currently produced via two different routes. The first route involves the use of p-nitrochlorobenzene. This route uses chlorine gas and generates chloride salts as a major waste product. The second route involves the nitrosation of aniline followed by thermal rearrangement. This route requires hydrogen chloride, produces a mixture of isomers, and produces nitrosoamines and benzidine as by-products.
A nonhalide route to substituted aromatic amines having at least two --NH.sub.2 groups, which could be utilized in the preparation of the corresponding isocyanates or carbamate esters, would provide significant advantages over current commercial technology and result in a more efficient and economic commercial process.
A nonphosgene process for preparing the isocyanates or carbamate esters of the invention which is economical and commercially viable, and includes the preparation of substituted aromatic amines having at least two --NH.sub.2 groups via a nonhalide route is highly desirable.